A solar cell may be defined as a device for converting light energy into electric energy by using a photovoltaic effect where electrons are produced by exposing a p-n junction diode to light. Such solar cells may be classified into a silicon solar cell, a compound semiconductor solar cell including group I-III-VI or group III-V, a dye-sensitized solar cell, and an organic solar cell according to a material used as a junction diode.
A solar cell made from CIGS (CuInGaSe), which is one of group I-III-VI Chal-copyrite-based compound semiconductors, represents superior light absorption, higher photoelectric conversion efficiency with a thin thickness, and superior electro-optic stability, so the CIGS solar cell is spotlighted as a substitute for a conventional silicon solar cell.
Different from a bulk solar cell, the CIGS thin film solar cell includes a plurality of unit cells, which are connected with each other in series through patterning processes (TH1 to TH3). The most important patterning process is the TH2 process. A connection wire makes contact with the back electrode at the TH2 pattern, so electric loss may occur and the efficiency of the solar cell may be significantly degraded if the contact fails at the TH2 pattern.